1. Field of the Invention
This invention is related to the field of probe-based nucleic acid sequence detection, analysis and quantitation. More specifically, this invention relates to novel compositions and methods pertaining to PNA Molecular Beacons.
2. Description of the Related Art
Quenching of fluorescence signal can occur by either Fluorescence Resonance Energy Transfer “FRET” (also known as non-radiative energy transfer: See: Yaron et al., Analytical Biochemistry 95: 228-235 (1979) at p. 232, col. 1, lns. 32-39) or by non-FRET interactions (also known as radiationless energy transfer; See: Yaron et al., Analytical Biochemistry 95 at p. 229, col. 2, lns. 7-13). The critical distinguishing factor between FRET and non-FRET quenching is that non-FRET quenching requires short range interaction by “collision” or “contact” and therefore requires no spectral overlap between the moieties of the donor and acceptor pair (See: Yaron et al., Analytical Biochemistny 95 at p. 229, col. 1, lns. 22-42). Conversely, FRET quenching requires spectral overlap between the donor and acceptor moieties and the efficiency of quenching is directly proportional to the distance between the donor and acceptor moieties of the FRET pair (See: Yaron et al., Analytical Biochemistny 95 at p. 232, col. 1, In. 46 to col. 2, In. 29). Extensive reviews of the FRET phenomenon are described in Clegg, R. M., Methods Enzymol., 221: 353-388 (1992) and Selvin, P. R., Methods Enzymol., 246: 300-334 (1995). Yaron et al. also suggested that the principles described therein might be applied to the hydrolysis of oligonucleotides (See: Yaron et al., Analytical Biochemistry 95 at p. 234, col. 2, lns. 14-18).
The FRET phenomenon has been utilized for the direct detection of nucleic acid target sequences without the requirement that labeled nucleic acid hybridization probes or primers be separated from the hybridization complex prior to detection (See: Livak et al. U.S. Pat. No. 5,538,848). One method utilizing FRET to analyze Polymerase Chain Reaction (PCR) amplified nucleic acid in a closed tube format is commercially available from Perkin Elmer. The TaqMan™ assay utilizes a nucleic acid hybridization probe that is labeled with a fluorescent reporter and a quencher moiety in a configuration that results in quenching of fluorescence in the intact probe. During the PCR amplification, the probe sequence specifically hybridizes to the amplified nucleic acid. When hybridized, the exonuclease activity of the Taq polymerase degrades the probe thereby eliminating the intramolecular quenching maintained by the intact probe. Because the probe is designed to hybridize specifically to the amplified nucleic acid, the increase in fluorescence intensity of the sample, caused by enzymatic degradation of the probe, can be correlated with the activity of the amplification process.
Nonetheless, this method preferably requires that each of the fluorophore and quencher moieties be located on the 3′ and 5′ termini of the probe so that the optimal signal to noise ratio is achieved (See: Nazarenko et al., Nucl. Acids Res. 25: 2516-2521(1997) at p. 2516, col. 2, lns. 27-35). However, this orientation necessarily results in less than optimal fluorescence quenching because the fluorophore and quencher moieties are separated in space and the transfer of energy is most efficient when they are close. Consequently, the background emission from unhybridized probe can be quite high in the TaqMan™ assay (See: Nazarenko et al., Nucl. Acids Res. 25: at p. 2516, col. 2, lns. 36-40).
The nucleic acid Molecular Beacon is another construct which utilizes the FRET phenomenon to detect target nucleic acid sequences (See: Tyagi et al. Nature Biotechnology, 14: 303-308 (1996). A nucleic acid Molecular Beacon comprises a probing sequence embedded within two complementary arm sequences (See: Tyagi et al, Nature Biotechnology, 14: at p. 303, col. 1, lns. 22-30). To each termini of the probing sequence is attached one of either a fluorophore or quencher moiety. In the absence of the nucleic acid target, the arm sequences anneal to each other to thereby form a loop and hairpin stem structure which brings the fluorophore and quencher together (See: Tyagi et al., Nature Biotechnology, 14: at p. 304, col. 2, lns. 14-25). When contacted with target nucleic acid, the complementary probing sequence and target sequence will hybridize. Because the hairpin stem cannot coexist with the rigid double helix that is formed upon hybridization, the resulting conformational change forces the arm sequences apart and causes the fluorophore and quencher to be separated (See: Tyagi et al. Nature Biotechnology, 14: at p. 303, col. 2, lns. 1-17). When the fluorophore and quencher are separated, energy of the donor fluorophore does not transfer to the acceptor moiety and the fluorescent signal is then detectable. Since unhybridized “Molecular Beacons” are non-fluorescent, it is not necessary that any excess probe be removed from an assay. Consequently, Tyagi et al. state that Molecular Beacons can be used for the detection of target nucleic acids in a homogeneous assay and in living cells. (See: Tyagi et al., Nature Biotechnology, 14: at p. 303, col. 2; lns. 15-77).
The arm sequences of the disclosed nucleic acid Molecular Beacon constructs are unrelated to the probing sequence (See: Tyagi et al., Nature Biotechnology, 14: at p. 303, col. 1; In. 30). Because the Tyagi et al. Molecular Beacons comprise nucleic acid molecules, proper stem formation and stability is dependent upon the length of the stem, the G:C content of the arm sequences, the concentration of salt in which it is dissolved and the presence or absence of magnesium in which the probe is dissolved (See: Tyagi et al., Nature Biotchology, 14: at p. 305, col. 1; lns. 1-16). Furthermore, the Tyagi et al. nucleic acid Molecular Beacons are susceptible to degradation by endonucleases and exonucleases.
Upon probe degradation, background fluorescent signal will increase since the donor and acceptor moieties are no longer held in close proximity. Therefore, assays utilizing enzymes known to have nuclease activity, will exhibit a continuous increase in background fluorescence as the nucleic acid Molecular Beacon is degraded (See: FIG. 7 in Tyagi et al: the data associated with (◯) and (□) demonstrates that the fluorescent background, presumably caused by probe degradation, increases with each amplification cycle.) Additionally, Molecular Beacons will also, at least partially, be degraded in living cells because cells contain active nuclease activity.
The constructs described by Tyagi et al. are more broadly described in WO95/13399 (hereinafter referred to as “Tyagi2 et al.” except that Tyagi2 et al. also discloses that the nucleic acid Molecular Beacon may also be bimolecular wherein they define bimolecular as being unitary probes of the invention comprising two molecules (e.g. oligonucleotides) wherein half, or roughly half, of the target complement sequence, one member of the affinity pair and one member of the label pair are present in each molecule (See: Tyagi2 et al., p. 8, ln. 25 to p. 9, ln. 3). However, Tyagi2 et al. specifically states that in designing a unitary probe for use in a PCR reaction, one would naturally choose a target complement sequence that is not complementary to one of the PCR primers (See: Tyagi2 et al., p. 41, ln. 27). Assays of the invention include real-time and end point detection of specific single-stranded or double stranded products of nucleic acid synthesis reactions, provided however that if unitary probes will be subjected to melting or other denaturation, the probes must be unimolecular (See: Tyagi2 et al., p. 37, lns. 1-9). Furthermore, Tyagi2 et al. stipulate that although the unitary probes of the invention may be used with amplification or other nucleic acid synthesis reactions, bimolecular probes (as defined in Tyagi2 et al.) are not suitable for use in any reaction (e.g. PCR) wherein the affinity pair would be separated in a target-independent manner (See: Tyagi2 et al., p. 13, lns. 9-12). Neither Tyagi et al. nor Tyagi2 et al. disclose, suggest or teach anything about PNA.
In a more recent disclosure, modified hairpin constructs which are similar to the Tyagi et al. nucleic acid Molecular Beacons, but which are suitable as primers for polymerase extension, have been disclosed (See: Nazarenko et al., Nucleic Acids Res. 25: 2516-2521(1997)). A method suitable for the direct detection of PCR-amplified DNA in a closed system is also disclosed. According to the method, the Nazarenko et al. primer constructs are, by operation of the PCR process, incorporated into the amplification product. Incorporation into a PCR amplified product results in a change in configuration that separates the donor and acceptor moieties. Consequently, increases in the intensity of the fluorescent signal in the assay can be directly correlated with the amount of primer incorporated into the PCR amplified product. The authors conclude, this method is particularly well suited to the analysis of PCR amplified nucleic acid in a closed tube format.
Because they are nucleic acids, the Nazarenko et al. primer constructs are admittedly subject to nuclease digestion thereby causing an increase in background signal during the PCR process (See: Nazarenko et al., Nucleic Acids Res. 25: at p. 2519, col. 1; lns. 28-39). An additional disadvantage of this method is that the Molecular Beacon like primer constructs must be linearized during amplification (See: Nazarenko et al., Nucleic Acids Res. 25: at p. 2519, col. 1, lns. 7-8). Consequently, the polymerase must read through and dissociate the stem of the hairpin modified Molecular Beacon like primer construct if fluorescent signal is to be generated. Nazarenko et al. does not suggest, teach or disclose anything about PNA.
In still another application of FRET to target nucleic acid sequence detection, doubly labeled fluorescent oligonucleotide probes which have been rendered impervious to exonuclease digestion have also been used to detect target nucleic acid sequences in PCR reactions and in-situ PCR (See: Mayrand, U.S. Pat. No. 5,691,146). The oligonucleotide probes of Mayrand comprise a fluorescer (reporter) molecule attached to a first end of the oligonucleotide and a quencher molecule attached to the opposite end of the oligonucleotide (See: Mayrand, Abstract). Mayrand suggests that the prior art teaches that the distance between the fluorophore and quencher is an important feature which must be minimized and consequently the preferred spacing between the reporter and quencher moieties of a DNA probe should be 6-16 nucleotides (See: col. 7, lns. 8-24). Mayrand, however teaches that the reporter molecule and quencher moieties are preferably located at a distance of 18 nucleotides (See: col. 3, lns. 35-36) or 20 bases (See: col. 7, lns. 25-46) to achieve the optimal signal to noise ratio. Consequently, both Mayrand and the prior art cited therein teach that the detectable properties of nucleic acid probes (DNA or RNA) comprising a fluorophore and quencher exhibit a strong dependence on probe length.
Resistance to nuclease digestion is also an important aspect of the invention (See: U.S. Pat. No. 5,691,146 at col. 6, lns. 42-64) and therefore, Mayrand suggests that the 5′ end of the oligonucleotide may be rendered impervious to nuclease digestion by including one or more modified internucleotide linkages onto the 5′ end of the oligonucleotide probe (See: U.S. Pat. No. 5,691,146 at col. 6, lns. 45-50). Furthermore, Mayrand suggests that a polyamide nucleic acid (PNA) or peptide can be used as a nuclease resistant linkage to thereby modify the 5′ end of the oligonucleotide probe of the invention and render it impervious to nuclease digestion (See: U.S. Pat. No 5,691,146 at col. 6, lns. 53-64). Mayrand does not however, disclose, suggest or teach that a PNA probe construct might be a suitable substitute for the practice of the invention despite having obvious knowledge of its existence. Furthermore, Mayrand does not teach one of skill in the art how to prepare and/or label a PNA with the fluorescer or quencher moieties.
The efficiency of energy transfer between donor and acceptor moieties as they can be influenced by oligonucleotide length (distance) has been further examined and particularly applied to fluorescent nucleic acid sequencing applications (See: Mathies et al., U.S. Pat. No. 5,707,804). Mathies et al. states that two fluorophores will be joined by a backbone or chain where the distance between the two fluorophores may be varied (See: U.S. Pat. No. 5,707,804 at col. 4, lns. 1-3). Thus, the distance must be chosen to provide energy transfer from the donor to the acceptor through the well-known Foerster mechanism (See: U.S. Pat. No. 5,707,804 at col. 4, lns. 7-9). Preferably about 3-10 nucleosides separate the fluorophores of a single stranded nucleic acid (See: U.S. Pat. No. 5,707,804 at col. 7, lns. 21-25). Mathies et al. does not suggest, teach or disclose anything about PNA.
From the analysis of DNA duplexes is has been observed that: 1: the efficiency of FET (or FRET as defined herein) appears to depend somehow on the nucleobase sequence of the oligonucleotide; 2: donor fluorescence changes in a manner which suggests that dye-DNA interactions affect the efficiency of FET; and 3: the Forster equation does not quantitatively account for observed energy transfer and therefore the length between donor and acceptor moieties attached to oligonucleotides cannot be quantitated, though it can be used qualitatively (See: Promisel et al., Biochemistry, 29: 9261-9268 (1990). Promisel et al. suggest that non-Forster effects may account for some of their observed but otherwise unexplainable results (See: Promisel et al., Biochemistry, 29: at p. 9267, col. 1, ln. 43 to p. 9268, col. 1, ln. 13). The results of Promisel et al. suggest that the FRET phenomena when utilized in nucleic acids in not entirely predictable or well understood. Promisel et al. does not suggest, teach or disclose anything about PNA and, in fact, the manuscript predates the invention of PNA.
The background art thus far discussed does not disclose, suggest or teach anything about PNA oligomers to which are directly attached a pair of donor and acceptor moieties. In fact, the FRET phenomenon as applied to the detection of nucleic acids, appears to be confined to the preparation of constructs in which the portion of the probe which is complementary to the target nucleic acid sequence is itself comprised solely of nucleic acid.
FRET has also been utilized within the field of peptides. (See: Yaron et al. Analytical Biochemistry 95 at p. 232, col. 2, ln. 30 to p. 234, col. 1, ln. 30). Indeed, the use of suitably labeled peptides as enzyme substrates appears to be the primary utility for peptides which are labeled with donor and acceptor pairs (See: Zimmerman et al., Analytical Biochemistry, 70: 258-262 (1976), Carmel et al., Eur. J. Biochem., 73: 617-625 (1977), Ng et al., Analytical Biochemistry, 183: 50-56 (1989), Wang et al., Tett. Lett., 31: 6493-6496 (1990) and Meldal et al., Analytical Biochemistry, 195: 141-147 (1991). Early work suggested that quenching efficiency of the donor and acceptor pair was dependent on peptide length (See: Yaron et al., Analytical Biochemistry 95 at p. 233, col. 2, lns. 36-40). However, the later work has suggested that efficient quenching was not so dependent on peptide length (See: Ng et al., Analytical Biochemistry, 183: at p. 54, col. 2, ln 23 to p. 55, col. 1, ln. 12; Wang et al., Tett. Lett., 31 wherein the peptide is eight amino acids in length; and Meldal et al. Analytical Biochemistry, 195 at p. 144, col. 1, lns. 33-37). It was suggested by Ng et al. that the observed quenching in long peptides might occur by an as yet undetermined mechanism (See: Ng et al., Analytical Biochemistry 183 at p. 55, col. 1, ln 13 to col. 2, ln 7.)
Despite its name, peptide nucleic acid (PNA) is neither a peptide, a nucleic acid nor is it even an acid. Peptide Nucleic Acid (PNA) is a non-naturally occurring polyamide (pseudopeptide) which can hybridize to nucleic acid (DNA and RNA) with sequence specificity (See U.S. Pat. No. 5,539,082 and Egholm et al., Nature 365: 566-568 (1993)). PNAs are synthesized by adaptation of standard peptide synthesis procedures in a format which is now commercially available. (For a general review of the preparation of PNA monomers and oligomers please see: Dueholm et al., New J. Chem., 21: 19-31 (1997) or Hyrup et. al., Bioorganic & Med. Chem. 4: 5-23 (1996)). Alternatively, labeled and unlabeled PNA oligomers can be purchased (See: PerSeptive Biosystems Promotional Literature: BioConcepts, Publication No. NL612, Practical PNA, Review and Practical PNA, Vol. 1, Iss. 2)
Being non-naturally occurring molecules, PNAs are not known to be substrates for the enzymes which are known to degrade peptides or nucleic acids. Therefore, PNAs should be stable in biological samples, as well as have a long shelf-life. Unlike nucleic acid hybridization which is very dependent on ionic strength, the hybridization of a PNA with a nucleic acid is fairly independent of ionic strength and is favored at low ionic strength, conditions which strongly disfavor the hybridization of nucleic acid to nucleic acid (Egholm et al., Nature, at p. 567). The effect of ionic strength on the stability and conformation of PNA complexes has been extensively investigated (Tomac et al., J. Am. Chem. Soc. 118: 5544-5552 (1996)). Sequence discrimination is more efficient for PNA recognizing DNA than for DNA recognizing DNA (Egholm et al., Nature, at p. 566). However, the advantages in point mutation discrimination with PNA probes, as compared with DNA probes, in a hybridization assay appears to be somewhat sequence dependent (Nielsen et al., Anti-Cancer Drug Design 8: 53-65, (1993)). As an additional advantage, PNAs hybridize to nucleic acid in both a parallel and antiparallel orientation, though the antiparallel orientation is preferred (See: Egholm et al., Nature at p. 566).
Despite the ability to hybridize to nucleic acid in a sequence specific manner, there are many differences between PNA probes and standard nucleic acid probes. These differences can be conveniently broken down into biological, structural, and physico-chemical differences. As discussed in more detail below, these biological, structural, and physico-chemical differences may lead to unpredictable results when attempting to use PNA probes in applications were nucleic acids have typically been employed. This non-equivalency of differing compositions is often observed in the chemical arts.
With regard to biological differences, nucleic acids, are biological materials that play a central role in the life of living species as agents of genetic transmission and expression. Their in vivo properties are fairly well understood. PNA, on the other hand is recently developed totally artificial molecule, conceived in the minds of chemists and made using synthetic organic chemistry. It has no known biological function (i.e. native (unmodified) PNA is not known to be a substrate for any polymerase, ligase, nuclease or protease).
Structurally, PNA also differs dramatically from nucleic acid. Although both can employ common nucleobases (A, C, G, T, and U), the backbones of these molecules are structurally diverse. The backbones of RNA and DNA are composed of repeating phosphodiester ribose and 2-deoxyribose units. In contrast, the backbones of the most common PNAs are composed on N-[2-(aminoethyl)]glycine subunits. Additionally, in PNA the nucleobases are connected to the backbone by an additional methylene carbonyl moiety.
PNA is not an acid and therefore contains no charged acidic groups such as those present in DNA and RNA. Because they lack formal charge, PNAs are generally more hydrophobic than their equivalent nucleic acid molecules. The hydrophobic character of PNA allows for the possibility of non-specific (hydrophobic/hydrophobic interactions) interactions not observed with nucleic acids. Further, PNA is achiral, providing it with the capability of adopting structural conformations the equivalent of which do not exist in the RNA/DNA realm.
The unique structural features of PNA result in a polymer which is highly organized in solution, particularly for purine rich polymers (See: Dueholm et al., New J. Chem., 21: 19-31 (1997) at p. 27, col. 2, lns. 6-30). Conversely, a single stranded nucleic acid is a random coil which exhibits very little secondary structure. Because PNA is highly organized, PNA should be more resistant to adopting alternative secondary structures (e.g. a hairpin stem and/or loop).
The physico/chemical differences between PNA and DNA or RNA are also substantial. PNA binds to its complementary nucleic acid more rapidly than nucleic acid probes bind to the same target sequence. This behavior is believed to be, at least partially, due to the fact that PNA lacks charge on its backbone. Additionally, recent publications demonstrate that the incorporation of positively charged groups into PNAs will improve the kinetics of hybridization (See: Iyer et al., J. Biol. Chem. 270: 14712-14717 (1995)). Because it lacks charge on the backbone, the stability of the PNA/nucleic acid complex is higher than that of an analogous DNA/DNA or RNA/DNA complex. In certain situations, PNA will form highly stable triple helical complexes through a process called “strand displacement”. No equivalent strand displacement processes or structures are known in the DNA/RNA world.
Recently, the “Hybridization based screening on peptide nucleic acid (PNA) oligomer arrays” has been described wherein arrays of some 1000 PNA oligomers of individual sequence were synthesized on polymer membranes (See: Weiler et al., Nucl. Acids Res. 25: 2792-2799(1997)). Arrays are generally used, in a single assay, to generate affinity binding (hybridization) information about a specific sequence or sample to numerous probes of defined composition. Thus, PNA arrays may be useful in diagnostic applications or for screening libraries of compounds for leads which might exhibit therapeutic utility. However, Weiler et al. note that the affinity and specificity of DNA hybridization to immobilized PNA oligomers depended on hybridization conditions more than was expected. Moreover, there was a tendency toward non-specific binding at lower ionic strength. Furthermore, certain very strong binding mismatches were identified which could not be eliminated by more stringent washing conditions. These unexpected results are illustrative of the lack of complete understanding of these newly discovered molecules (i.e. PNA).
In summary, because PNAs hybridize to nucleic acids with sequence specificity, PNAs are useful candidates for investigation as substitute probes when developing probe-based hybridization assays. However, PNA probes are not the equivalent of nucleic acid probes in both structure or function. Consequently, the unique biological, structural, and physico-chemical properties of PNA requires that experimentation be performed to thereby examine whether PNAs are suitable in applications where nucleic acid probes are commonly utilized.